Compact, high voltage capacitors are utilized as energy storage reservoirs in many applications, including implantable medical devices. These capacitors are required to have a high energy density since it is desirable to minimize the overall size of the implanted device. This is particularly true of an Implantable Cardioverter Defibrillator (ICD), also referred to as an implantable defibrillator, since the high voltage capacitors used to deliver the defibrillation pulse can occupy as much as one third of the ICD volume.
Implantable Cardioverter Defibrillators, such as those disclosed in U.S. Pat. No. 5,131,388, incorporated herein by reference, typically use two electrolytic capacitors in series to achieve the desired high voltage for shock delivery. For example, an implantable cardioverter defibrillator may utilize two 350 to 400 volt electrolytic capacitors in series to achieve a voltage of 700 to 800 volts.
Electrolytic capacitors are used in ICDs because they have the most nearly ideal properties in terms of size, reliability and ability to withstand relatively high voltage. Conventionally, such electrolytic capacitors include an etched aluminum foil anode, an aluminum foil or film cathode, and an interposed kraft paper or fabric gauze separator impregnated with a solvent-based liquid electrolyte. While aluminum is the preferred metal for the anode plates, other metals such as tantalum, magnesium, titanium, niobium, zirconium and zinc may be used. A typical solvent-based liquid electrolyte may be a mixture of a weak acid and a salt of a weak acid, preferably a salt of the weak acid employed, in a polyhydroxy alcohol solvent. The electrolytic or ion-producing component of the electrolyte is the salt that is dissolved in the solvent. The entire laminate is rolled up into the form of a substantially cylindrical body, or wound roll, that is held together with adhesive tape and is encased, with the aid of suitable insulation, in an aluminum tube or canister. Connections to the anode and the cathode are made via tabs. Alternative flat constructions for aluminum electrolytic capacitors are also known, comprising a planar, layered, stack structure of electrode materials with separators interposed therebetween, such as those disclosed in the above-mentioned U.S. Pat. No. 5,131,388.
In ICDs, as in other applications where space is a critical design element, it is desirable to use capacitors with the greatest possible capacitance per unit volume. Since the capacitance of an aluminum electrolytic capacitor is provided by the anodes, a clear strategy for increasing the energy density in the capacitor is to minimize the volume taken up by paper and cathode and maximize the number of anodes. A multiple anode stack configuration requires fewer cathodes and paper spacers than a single anode configuration and thus reduces the size of the device. A multiple anode stack consists of a number of units consisting of a cathode, a paper spacer, two or more anodes, a paper spacer and a cathode, with neighboring units sharing the cathode between them. Energy storage density can be increased by using a multiple anode stack configuration element; however, the drawback is that the equivalent series resistance, ESR, of the capacitor increases as the conduction path from cathode to anode becomes increasingly tortuous. To charge and discharge the inner anodes (furthest from the cathode) charge must flow through the outer anodes. With typical anode foil, the path through an anode is quite tortuous and results in a high ESR for a multiple anode stack configuration. By keeping the ESR low, however, the charge efficiency and DSR (delivered to stored energy ratio) of the capacitor are maximized.
The conduction path from the cathode to the inner anodes may be made less tortuous by providing pores in the outer anode foil. In this manner, charge can flow directly through the outer anodes to the inner anodes. Thus, the use of porous anode foil can combat the increase in ESR resulting from the use of a multiple anode stack configuration. U.S. Pat. No. 6,802,954 to Hemphill et al., incorporated herein by reference in its entirety, describes an electrochemical drilling process for creating porous anode foil for use in multiple anode stack configuration electrolytic capacitors which produces a pore structure that is microscopic in pore diameter and spacing, allowing for increased energy density with a minimal increase in ESR of the capacitor. An etched foil is placed into an electrochemical drilling solution and a DC power supply is used to electrochemically etch the foil in the electrochemical drilling solution such that pores on the order of a few microns diameter are produced through the foil. The electrochemical drilling process creates large diameter “through” type tunnels, or pathways, in the foil that increase the electrical porosity of the foil, thereby improving charge efficiency and DSR. A widening process may be employed to widen tunnel diameter, maximizing surface area and reducing the taper of tunnels. A barrier layer oxide is formed on the anode foil, and the tunnel diameter should be large enough to leave a pore in the tunnels despite this oxide layer.
The electrochemical drilling process in accordance with U.S. Pat. No. 6,802,954, however, utilizes an electrochemical drilling solution with an initially neutral pH that becomes slightly basic with a pH of around about 9 to 11 shortly after starting processing of foils, causing aluminum dissolution in the solution to precipitate as aluminum hydroxide. This aluminum hydroxide solid should be filtered from solution if it is desired to process a plurality of foils; otherwise, the electrochemical drilling solution becomes less effective in creating “through” type tunnels that improve electrical porosity. Additionally, the aluminum hydroxide solid may build up on the process equipment, causing production downtime for routine cleaning (e.g., weekly) of the process equipment using a caustic solution. The electrochemical drilling solution should be dumped routinely (e.g., daily) and replaced with new solution free of solids.
What is needed, then, is a consistent and efficient method of creating a plurality of porous anode foil for use in capacitors that minimizes ESR while maintaining high capacitance.